The cardiac electrical signal begins in the cells of the sinoatrial node in the right atrium. These cells spontaneously depolarize and create a cardiac action potential of electrical impulses that rapidly propagates outward across the right atrium and then the left atrium. The cardiac action potential in turn stimulates muscle cells of the atrial myocardium to depolarize and contract to push blood into the ventricles. Shortly thereafter, this atrial action potential encounters the atrioventricular node located at the juncture of the atria and ventricles near the center of the heart. The atrioventricular node slightly delays cardiac action potential propagation into the ventricles to ensure complete drainage of blood from the atria. Thereafter, the muscle cells of the ventricular myocardium are activated by the electrical wave front and are stimulated into systolic contraction. After a rest and reset period, the complete the heart beat cycle repeats. Any disruption in this process, which can include heart block, sinus bradycardia, atrial fibrillation, and ventricular tachycardia, can lead to the symptoms ranging from dizziness to a sensation of heart fluttering or palpitations, loss of consciousness or even death. Being able to record the electrical signal of the heart is a fundamental diagnostic tool of every physician.
Identifying abnormal rhythms depends upon the manner in which and the amplitude of the depolarization signal of the muscle cells of the atrial and ventricular myocardium that in turn act as sequential voltage sources, which generate a current flow across the thoracic region of the body and result in a characteristic signal on the body surface. In a typical electrocardiographic (ECG) monitor, cardiac action potentials occur between 0.05 Hz to 150 Hz with a signal strength of around 3 mVp-p (peak-to-peak). Although miniscule, the current flow can be measured to characterize the electrical activity of the heart using an ECG monitor or similar device. Voltage differentials from pairings of the electrodes are filtered, amplified, and combined into P, QRS, and T complexes.
Conventionally, cardiac action potentials are detected through electrodes attached to the skin on the chest and limbs based on the American Heart Association's classic 12-lead placement model, such as P. Libby et al., “Braunwald's Heart Disease—A Textbook of Cardiovascular Medicine,” Chs. 11 and 12 (8th ed. 2008), the disclosure of which is incorporated by reference. Both traditional in-clinic and ambulatory Holter-style ECG monitors follow the standard 12-lead model with variations on numbers and placement of leads. Generally, limb lead electrodes are placed on each arm and on the left leg, while precordial lead electrodes are placed on the left upper chest region over the heart in close proximity to the heart and at a location of strongest cardiac action potential signal strength. In turn, the monitoring circuitry relies on the superior signal strength from over-the-heart electrode placement and the relatively long signal vector length that is afforded by lead placement over a wider physical expanse of the body. For instance, based upon the large inter-electrode distances, signal amplification assumes a signal strength of around 3 mVp-p (peak-to-peak). The limb leads can be re-positioned as necessary to compensate for variability in patient anatomy due to tissue and bone density and heart position.
The 12-lead placement model, however, is poorly suited to long-term ambulatory monitoring both from the perspective of comfort and from the perspective of reliability, particularly in adult women, as well as on other patients with large-girthed, fatty, or well-developed upper chest regions. The latter concern simply relates to how standard monitoring electrodes fall off with modest movement, as well as how signal quality diminishes when electrodes are placed over breast tissue, as is unavoidable in some women. In-clinic ECG monitoring, for instance, assumes that the patient will remain relatively stationary and that the limb leads can be repositioned as necessary to provide sufficient electrode separation for recording a signal of reasonable amplitude and to compensate for variability in patient anatomy. In contrast, during ambulatory monitoring, a woman's body is in continual motion, even during sleep, albeit to a lesser degree. Electrodes are apt to detach and signal quality degrades or is absent altogether.
Moreover, the strictly in-clinic nature of conventional 12-lead monitoring inherently compensates for differences in physical body size, as electrodes can be placed in their ordinary positions with electrical lead cables adjusting to anatomical differences. In ambulatory monitoring, however, physical characteristics of the human body play a major role in electrode adhesion and signal capture, both of which have a direct impact on long-term monitoring quality and efficacy. Physical characteristics vary significantly from patient to patient, and even for the same patient over time. For instance, bone structure, musculature, and fat tissue in the thoracic region all affect the aggregate density of body mass due to physiological differences in body type, gender, age, physical constitution, and posture. As well, bone density and musculature tends to drastically decrease in geriatric patients as a result of the natural ageing process. Increased body mass density increases signal impedance and noise. An ambulatory ECG monitor with electrode leads integrated into a unitary device would be impracticable, as one package size would not fit all patients, whereas using wired leads connected to a separate control unit increases patient discomfort, while adding complexity and decreasing reliability.
Additionally, in women, changes in body position, for instance, lying down, stretching, or bending over, can displace the positioning of the breasts and the corresponding changes in tissue and bone density can deleteriously affect any electrodes placed thereon. Breasts also exhibit pendulous motion in proportion to overall size in response to motor activities, such as walking, running, biking, or exercise. Such recurrent motion can act to progressively detach items adhered to the soft tissues, like the breasts, and are likely to irritate the skin when motion leads to electrode patch tension. Moreover, breast tissue can increases the distance between sensing electrodes placed and the underlying heart. Breast tissue may also force placement of the electrode in a suboptimal location for recording the cardiac signal to remain comfortable, especially during long-term monitoring. The trade-off in women, especially active or large breasted, buxom women, can account for poor ECG signal quality.
Notwithstanding, Holter and other forms of ambulatory ECG monitors generally still rely on electrodes placed close to the heart as suggested by the 12-lead placement model. For instance, U.S. Pat. No. 3,215,136 issued Nov. 2, 1965 to Holter et al. discloses an electrocardiographic recording and playback means. Episodes of ventricular tachycardia, asystolic intervals, and ectopic heart activities are sensed by electrodes disposed on the patient's skin in a suitable location, with sufficient inter-electrode separation. These signals are ordinarily recorded via a compact recorder worn by the patient that records an electrocardiogram (ECG) while he engages in activities of daily living, which subsequently allows a cardiac specialist to temporally correlate patient symptoms and cardiac abnormalities with activities. A cardiac rhythm disorder, as well as the absence of a rhythm disorder during symptoms, can sometimes be identified by having the patient record those symptoms during the use of the Holter monitor.
U.S. Pat. No. 6,117,077 issued Sep. 12, 2000 to Del Mar et al. discloses a long-term ambulatory physiological recorder provided in a relatively planar and triangular-shaped recorder housing with three adhesive electrode pads. The recorder is fully self-contained and mounted immediately adjacent to the organ system that is to be monitored. Electrode pads are adhesively and conductively attached to the patient's left chest in a position generally over the heart with positive and negative terminals in a relative vertical position from the top to the bottom of the heart. Additional electrode leads can also be connected to an input port on the recorder and placed over adjacent areas of the upper chest.
U.S. Pat. No. 6,456,872 issued Sep. 24, 2002 to Faisandier discloses a Holter-type apparatus for recording physiological signals indicative of cardiac activity. A base unit is formed of a flexible sheet carrying electrodes and a recording case that carries a battery and flexible printed circuit material. The base unit is disposable and can be changed with each new patient examination. The recorder case is fixed in position on the patient's thorax through a plurality of electrodes affixed either through adhesion or through depression using suction cups. Alternatively, the base unit can be carried by a thoracic belt or a hanging strap collar. The recording case includes electronic circuits for the collection and processing of ECG signals and a data transmission port is provided for by-directional exchange of data, control parameters, and information.
U.S. Pat. No. 7,257,438 issued Aug. 14, 2007 to Kinast discloses a patient-worn medical monitoring device that includes a lanyard and electronics package supported in the manner of a pendant. A lanyard includes integral electrodes or other sensors for making physiological measurements, which may be stored in a monitor for later readout or transmitted, before or after processing, to a remote location. The device can locally process and analyze a patient's signals and transmit only summary data or analyzed results to a remote device.
Finally, U.S. Patent application, Publication No. 2007/0255153, filed Nov. 1, 2007, to Kumar et al.; U.S. Patent application, Publication No. 2007/0225611, filed Feb. 6, 2007, to Kumar et al.; and U.S. Patent application, Publication No. 2007/0249946, filed Feb. 6, 2007, to Kumar et al. disclose discloses a non-invasive cardiac monitor and methods of using continuously recorded cardiac data. A heart monitor suitable for use in primary care includes a self-contained and sealed housing. The housing encloses an electronic memory connected to electrodes on the upper left chest to detect an ECG. A thin, flexible, and tapered rim or lip is provided around the edges of the electronics portion of the monitor to increase the surface area available for adhesion. Continuously recorded cardiac monitoring is provided through a sequence of simple detect-store-offload operations that are performed by a state machine. The housing is adapted to remain affixed to a patient for at least seven days. The heart monitor can include an activation or event notation button, the actuation of which increases the fidelity of the ECG information stored in the memory. The stored information can be retrieved and analyzed offline to identify both normal and abnormal ECG events. The monitor is specifically intended to provide monitoring continuously and without interruption over an extended period. Despite the improvement in size and ease of use of such a system, neither this device or any of the above described systems defines a device capable of extremely simple and reliable application for any body habitus and by any individual regardless of training. The application of this monitor is especially problematic for large breasted, buxom women.
Finally, U.S. Patent application, Publication No. 2008/0284599, filed Apr. 28, 2006, to Zdeblick et al. and U.S. Patent application, Publication No. 2008/0306359, filed Dec. 11, 2008, to Zdeblick et al., disclose a pharma-informatics system for detecting the actual physical delivery of a pharmaceutical agent into a body. An integrated circuit is surrounded by pharmacologically active or inert materials to form a pill, which dissolve in the stomach through a combination of mechanical action and stomach fluids. As the pill dissolves, areas of the integrated circuit become exposed and power is supplied to the circuit, which begins to operate and transmit a signal that may indicate the type, A signal detection receiver can be positioned as an external device worn outside the body with one or more electrodes attached to the skin at different locations. The receiver can include the capability to provide both pharmaceutical ingestion reporting and psychological sensing in a form that can be transmitted to a remote location, such as a clinician or central monitoring agency.
Therefore, a need remains for an ambulatory ECG monitoring device and method of use adapted to long term monitoring that resists body movement while providing ease and discreteness of use and patient comfort regardless of patient knowledge and regardless of patient body habitus. Additionally, such an ambulatory ECG monitoring device and method of use would preferably adapt to a wide range of different body characteristics and physiques, particularly ample-breasted women and other individuals with significant breast tissue.